Hydrostatic or “hystat” drive refers generally to a drive train or portion of a drive train in a work machine utilizing hydraulic fluid pressurized by engine rotation as the motive force for propelling the work machine. In a typical design, a pump is driven with an output shaft of the engine and provides pressurized hydraulic fluid to a hydraulic motor, in turn coupled with one or more axles of the work machine. In most hystat drive work machines, the pump and motor each have a variable displacement, allowing the relative torque and speed of a drive shaft to be continuously varied.
Where it is desirable to provide a relatively higher torque to the work machine wheels or tracks, the displacement of the motor will be relatively large such that at a given hydraulic pressure from the pump, a relatively large force is transferred to the drive shaft and wheels or tracks for each stroke of the motor. Similarly, where it desirable to provide a relatively lower torque to the work machine wheels or tracks, for example, when operating the work machine at relatively higher velocity, the relative displacement of the motor can be decreased. Displacement of the pump can be similarly adjusted, creating a continuously variable coupling between the engine and the ground engaging wheels or tracks of the work machine.
While the combination of a variable displacement pump and variable displacement motor in a hystat work machine imparts tremendous flexibility in operation, there is room for improvement. Particularly in work machine applications that require relatively frequent accelerations, decelerations, stops and starts, for example, loader operations, the wear and tear on the work machine and even the operator can be significant.
In some known hydrostatic drive designs, operators can slow the work machine by decreasing the pump displacement. This approach takes advantage of available energy in the form of hydraulic pressure in the drive system that might otherwise be wasted. In effect, the pump acts as a hydraulic brake. Moreover, using the pump to slow the work machine can somewhat reduce the use of wheel brakes, prolonging their life. In many instances, however, the pumps mounted in hydrostatic drive work machines are insufficiently large or have too slow a stroking rate to substantially slow the work machine in a reasonable amount of time. Larger pumps having this capability tend to be significantly more expensive.
Even where a sufficiently large or fast pump is available, it can be challenging for even highly skilled operators to adjust the pump at an appropriate rate. Changing pump displacement too quickly could cause the work machine to decelerate too rapidly, or can unduly change the rate of deceleration, such that the work machine and operator are jerked. This can be not only uncomfortable for the operator, but can also risk tipping the machine or spilling a load carried thereon. On the other hand, changing pump displacement too slowly can fail to bring the work machine to a stop quickly enough and can waste operating time. The operator's control can thus limit shift quality, for example, if the operator adjusts the pump too slowly, or compromise smoothness, for example, if the pump is adjusted too quickly or the wheel brakes are applied too forcefully. Moreover, different operators can have different personal shifting preferences. While one operator may be willing to undergo a relatively abrupt stop, other operators may be unwilling or unable to tolerate jerks or rapid decelerations. Thus, hardware selection alone is typically inadequate in ensuring a system well suited to different operators.
Co-owned U.S. Pat. No. 5,624,339 shows a method for controlling shift points in a continuously variable transmission that includes a hydrostatic drive path or a combined hydrostatic and mechanical transmission drive path. The mechanical transmission includes a planetary summing arrangement that appears to allow for smooth shift without disruption of torque. Although this strategy and structure appears promising, there always remains room for improving upon the overall combination of work efficiency with rider comfort.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.